C1q CIC in Lupus Nephritis: An Analysis of 883 Patients.

Objectives: Lupus nephritis (LN) is a major complication of systemic lupus erythematosus (SLE). This study evaluated the association of C1q-circulating immune complexes (C1q CIC) with LN and their potential adjunctive rule-out value compared with conventional biomarkers. Methods: We conducted a retrospective analysis of 883 SLE patients from the Asia Pacific Lupus Collaboration (APLC) cohort in Taiwan. C1q CIC was assessed for its association with disease activity and compared to anti-dsDNA, C3, and C4. Logistic regression and ROC analysis were performed. Results: C1q CIC levels were significantly elevated in active SLE and in non-renal manifestations, including cutaneous involvement, serositis, and hematologic abnormalities. In patients with LN, C1q CIC levels were significantly higher than in those without LN, whereas anti-dsDNA, C3, and C4 showed no significant differences. Overall discriminative performance was modest, and C1q CIC is not suitable as a diagnostic or screening tool for LN. At a cutoff of 5.31 μg Eq/mL, C1q CIC showed a high negative predictive value (NPV = 86.04%) but low positive predictive value (PPV = 29.51%). In stratified analysis, discriminative ability was limited in patients with low disease activity (SLEDAI-2K < 4; AUC = 0.55), whereas NPV remained high (97.15%). Conclusion: C1q CIC may serve as an adjunctive rule-out biomarker for LN, particularly when levels are low or in patients with low disease activity. Although not suitable for diagnosis or screening, its relatively high NPV may help identify patients at lower risk of LN. Results should be interpreted in conjunction with clinical and laboratory assessment.